Sunday, January 28, 2018

Saturday Night's Rime "Storm"

A thin veil of altostratus clouds led to an upper-elevation riming event Saturday night that likely threw a wrench at some lift operations in the central Wasatch this morning (Sugarloaf was a late start at Alta, although I'm not sure if that was due to rime or a mechanical).

The large-scale setup was typical for these events in the Wasatch Mountains with moist, northwesterly flow spillover over a ridge located near the Pacific coast.

This mornings sounding shows very dry conditions at low levels over the Salt Lake Valley, but saturated conditions, indicated by the colocated dewpoint (green) and temperature (red) traces within the cloud layer from about 725 to 650 mb.  This is consistent with cloud bases near about 10,000 ft.

Source: SPC
The key for rime is that the clouds need to be below 0ºC, but not have cloud top temperatures much below about -8 to -10ºC.  Indeed, this mornings sounding suggests a cloud-top temperature around -10ºC.  At such temperatures, the cloud droplets can exist in large concentrations in liquid form despite the below "freezing" temperatures.  These drops then freeze on contact with whatever object that they meet.  Note also that the winds at 700-mb are about 30 knots.  Flow is needed for significant riming to occur since it's the main mechanism for transporting the cloud droplets.  Without it, you'll probably get mostly hoar frost.  If cloud tops are colder, enough ice can form in the cloud to "glaciate" (i.e., freeze) enough of the supercooled cloud droplets that riming is limited.

In some rime events, larger cloud or drizzle drops can form.  That may have been the case last night.  The Supreme Chair, for example, was coated in a fairly clear layer of ice that suggests to me some larger drops may have around.

The situation should be better tomorrow.  Overnight, as the ridge shifts eastward, the flow weakens and the mid levels dry, as can be seen in the time-height section below.

That should make for a pretty day too.

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